The task of testing data transmission systems has become more and more challenging as the versatility of data transmission systems and connected protocols and applications has increased. In order to be able to test the different functions of a data transmission system, it is necessary to be able to model the variation in the transmission delay caused by the data transmission network between various network elements, such as routers. Thus, it is necessary to set a desired probability distribution between the inter-packet intervals of successive data frames for the test traffic arriving in the network element.
The test traffic can be based for example on the data traffic measured in a real data transmission network. The measurement result can be represented for example as a file where there is recorded, in chronological order, the arrival time and size of each data frame that has arrived in the measurement location. In that case the measured size values and arrival times unambiguously define the size distribution of the data frames in the test traffic, as well as the distribution of time intervals between successive data frames. In a testing situation, the test traffic between the network elements to be tested is delayed by an adjustable delay element so that the time intervals between successive data frames are defined on the basis of values read from said file. Another generally known method for adjusting the delay of test traffic is to produce the values representing the time interval between successive data frames by using a random number generator and a pre-determined probability distribution. An apparatus that models the delay fluctuation in a data transmission network must process the test traffic so that the time intervals between successive data frames sent from said apparatus represent the desired distribution at a sufficient accuracy. The correctness of the time interval distribution is particularly important when testing for example the transmission of timing information over a data transmission network and the quality of Circuit Emulation Service, CES.